Bend Water with an Electric Field
If you've played with magnets, you know that a magnet can be used to attract some metal objects. This might have seemed like magic at first, but it is actually due to the magnetic field that is produced produced by the magnet. In this experiment you will instead use an electric field- created by simply combing your hair- to attract a stream of water! The magic of science!
What you'll need:
- Water faucet (bet you have one in your bathroom)
- Plastic comb
- Your hair
Basic Experimental Procedure (also watch the video below):
- Open the water faucet to produce the thinnest stream of water you can without it breaking up into drops.
- Run the comb through your hair several times.
- Hold the comb near the stream of water (but don't let it get wet!) and watch the water bend towards the comb.
- Hold the comb on the other side of the stream. It still bends toward the comb.
- Move the comb nearer or farther from the stream. How does that affect the bending of the water?
Everything is made up of matter, matter is made of atoms (which can combine to form molecules), and atoms are made up of protons, neutrons and most importantly for this experiment, electrons. The protons and neutrons are stuck together in the nucleus of the atom, while the much, much tinier electrons fly around the nucleus. It's kind of like the way planets orbit around the sun (the electrons are like the planets and the nucleus is like the sun), but electrons don't follow orderly paths like the planets do. What keeps the electrons from flying away from the nucleus is a force called the electric force. We say that electrons and protons have an electric charge, or just charge (neutrons don't have any charge- they're neutral). Electrons have a negative charge (-), protons have a positive charge (+), and charged objects create an electric field that can attract or repel other charged objects. Opposite charges attract each other and the closer they are to each other, the stronger the field and the force of attraction. Similar charges (such as two electrons or two protons) repel each other (the closer they are the stronger the field and repulsive force). [BTW- If this reminds you of the way magnets can attract or repel each other depending on their north or south poles and how close they are to each other, you're right. Electric and magnetic forces (and fields) are actually related to each other, and often called the electromagnetic force (and electromagnetic field). This is one of the four fundamental forces in nature (see the link below to learn more about this and the other fundamental forces that hold atoms together).]
Different types of atoms (called elements) and the molecules they combine to form may be stronger or weaker when it comes to attracting and holding onto their electrons. In our case, the molecules that make up the plastic comb hold onto their electrons much more strongly than the molecules which make up your hairs hold onto theirs. When you comb your hair, some of the comb's molecules get very close to some of the hair's molecules- so close, in fact, that the atoms in the comb actually start attracting or pulling on the electrons attached to the atoms in the hair. Likewise, the hair's atoms also start pulling on the comb's electrons. It's kind of a tug-of-war battle, but with electrons (using the electric field force) instead of rope. Normally most molecules have the same number of negative and positive charges, (so we say they are neutral), but when you take the comb away from your hair some of the hair's electrons are actually pulled away, sticking to the comb's atoms instead (remember that the comb's atoms are stronger at attracting electrons). Thus the comb now has extra electrons- and extra electric charges- and youur hairs have too few electrons. We say the comb is negatively charged and your hair is positively charged. That's why your hairs are attracted or stand up if you bring the comb close to your head (opposite charges attract). The more you comb your hair, the more charges build up, and the stronger the electric field (and attractive force) will be. Charging an object this way is called contact electrification, or the tribo-electric effect (learn more about this with the link below). This is also commonly called static electricity, but many scientists don't like this name, because electricity is defined as moving electric charges, thus static (non moving) electricity doesn't really make sense.
Water molecules are made up of two hydrogen atoms and one oxygen atom attached to form a triangle shape, kind of like Mickey Mouse's head- oxygen is the face and the two hydrogens are his ears. All of the electrons in these atoms move around in such a way that the oxygen corner of the triangle is more negatively charged and the hydrogen corners are more positively charged. This kind of molecule is called dipolar. Since water molecules in a liquid are free to move around and rotate, when you hold your negatively charged comb near a stream of water, the positive (hydrogen) sides of each water molecule are attracted to the comb's electric field and rotate so that they point towards the comb. This means that the side of the water stream closest to the comb is more positively charged and the opposite side of the stream is more negatively charged. Finally, the electric field of the negatively charged comb attracts (or pulls) on the positively charged side of the water stream, while it also repels (or pushes away) the opposite (negatively charged) side of the stream, but since the positive side is closer, the attractive force there is much stronger and the stream of water is pulled towards the comb. If you move the comb to the other side of the stream the water molecules simply rotate again to point the opposite direction, so that the side of the stream closest to the comb is once again more positive, and thus the stream still bends toward the comb.
By the way- if you hold a magnet near the stream of water nothing happens. It is neither attracted or repelled by the magnet. But that doesn't mean that water (or anything made mostly of water, like frogs or even people) can't be influenced by a magnetic field- you just need a really powerful magnetic field. In fact, if your magnet is strong enough you can even levitate a frog (see the link below)!
Variations and Related Activities:
You can also bend the water stream with a balloon or plastic rod (PVC pipe works very well). Just charge them up by rubbing with your hair, cloth (try different materials) or a piece of fur (read more about the tribo-electric series or different materials below). Try making the faucet drip instead of a continuous stream. Are the droplets attracted to the comb? And while you're in the bathroom anyway, take a small piece of toilet paper and dangle it vertically near your comb. It should also be attracted to the comb, because the comb's electric field induces a small electric dipole moment (like a charge) in the paper's molecules that is attracted to the comb. In fact, if you tear the toilet paper into really tiny pieces you can even use the charged comb to lift them right off a table or countertop.
Did you notice that if you get the comb wet, it no longer bends the water? The extra electrons that were stuck on the comb are able to move into the water and escape, thus the comb is no longer charged (no electric field). Also, if you wet the comb first, then run it through your hair, the water prevents it from grabbing electrons from your hair and charging up, so this doesn't bend a stream of water either. Even a microscopically thin layer of water on the comb, which can happen on a humid day, will prevent it from charging up (if you're having trouble making the experiment work, this may be the problem).
Here is a very different way of doing this experiment:
Links to more information and activities
Other's versions of this experiment:
More on contact electrification (static electricity):
The tribo-electric effect and tribo-electric series:
The fundamental forces within an atom:
Induced dipole moments or charges:
And just for fun, levitating a frog in a very strong magnetic field: